Cardiac surgical procedures are well known and the associated surgical techniques have progressed to the point where the risk to the patient has been greatly minimized. At the conclusion of cardiac surgery, it is typical for the surgeon to implant temporary pacing leads in the myocardium of the atria or ventricles of the cardiac patient's heart. The primary purpose of the temporary pacing leads is to provide a means for pacing the atria and ventricles. The pacing leads also provide a means for recording myocardial electrograms.
Conventional temporary pacing leads typically consist of a conductive wire lead and a surgical needle for implantation in the myocardium. The needle typically has a distal piercing end and a proximal end which is attached to the distal end of the wire lead. An electrode needle is typically attached to the proximal end of the wire lead. The wire lead typically has an insulated section and a distal conductive section where the insulation has been removed. The distal conductive section is implanted in the cardiac muscle. The electrode needle is typically pushed through the thoracic cavity wall ( i.e., from the interior to the exterior ) and the insulated section of the wire lead is partially pulled through. The electrode needle is connected to conventional pacing and monitoring equipment.
Conventional wire leads are typically made from multi-filament conducting materials such as stainless steel. The leads are insulated with bio-compatible polymer coatings . It is important that the wire have a feel to the surgeon similar to that of a conventional suture. This is known in this art as "hand".
Conventional cardiac pacing leads are typically used in the following manner. Initially, the surgeon inserts the curved surgical needle into the myocardium. Next, the surgeon pushes the needle through and out of the myocardium such that a portion of the uninsulated, distal conductive section of the pacing wire is implanted in a needle pathway formed in the myocardium by the surgical needle. The surgical needle is then cut from the pacing wire, and, the pacing wire is optionally anchored to the heart with a suture. Then, the surgeon pushes the scored electrode needle, connected to the proximal end of the pacing wire, out through the wall of the thoracic cavity so that the electrode needle and a length of the insulated section of electrode lead wire are positioned external to the chest of the patient. The electrode needle is then broken at the score line and inserted into the appropriate receptacle of a conventional electronic pacing and monitoring device. Typically, the leads are removed from the patient, after the patient has been stabilized, in the following manner. The surgeon disconnects the electrode needle from the monitoring/pacing device. The surgeon then grasps the external section of the pacing wire and pulls it by applying traction( i.e., an axial, rearward force), thereby pulling the lead from the myocardium and out through the chest wall.
It is known that pacing leads must have certain characteristics. First of all, the lead wire must be very flexible and resistant to fatigue failure resulting from repeated flexing cycles. It can be appreciated that a cardiac patient may have the pacing leads in place for periods of time up to and exceeding two or three weeks. During that time normal voluntary and involuntary movements by the patient( such as heartbeats, respiration and ambulation) will result in repetitive flexing of the wire lead. Another requirement for cardiac pacing leads is that the implanted myocardial lead must remain in place when subjected to typical stress and strain caused by typical patient movement and activities. However, the implanted lead must be easily withdrawn from the myocardium by the application of an axial force, i.e., traction, to the wire lead. Yet another requirement is that the pacing wire be constructed of material having a low electrical resistance. Additionally, the cardiac pacing leads should be easy to manufacture. A further requirement is that the cardiac leads must be sufficiently flexible to provide the surgeon with a "hand" similar to that of a conventional suture. Still another requirement of a pacing lead is biocompatability such that the pacing lead is corrosion resistant, nontoxic, and nonallergenic.
Conventional cardiac pacing leads are known to have deficiencies. Typically, conventional pacing leads use a multi-conductor(filament) wire having an outer electrically insulating coating such as the multi-filament stainless steel sutures disclosed in U.S. Pat. No. 3,125,095. A multi-filament pacing wire, although providing the required "hand", flexibility and resistance to fatigue, also allows the growth of tissue into the interstices of the wire in sections where the outer insulation has been removed. This tissue ingrowth complicates and may impeded removal of the pacing lead from the heart. Furthermore, from a manufacturing perspective, multi-filament wire is difficult to work with. For example, it is difficult to strip all of the insulation coating from a multi-filament wire using conventional wire stripping equipment. A residue of insulation in the interstices between the filaments is typically left behind. Any residual insulation may adversely affect pacing and monitoring. In addition, when using a multi-filament wire , it is necessary to simultaneously cut the wire and weld the ends of the strands. Cutting and welding must be done at the same time in order to prevent unraveling. It is known that when cut and welded ends are inserted into surgical needles for mounting it is possible for there to be deficient electrical contact between the cut and welded end of the wire and the needle. This poor electrical contact may be caused by oxidation or poor physical configuration resulting from the cutting and welding step.
The use of a single filament wire would eliminate some of these disadvantages. However, although single filament wires made from conventional metals may have been used for cardiac pacing leads, they have the disadvantages of not having the proper "hand" and of being susceptible to fatigue fracture or failure.
Another disadvantage of conventional pacing leads is that the pacing lead must somehow be anchored in the myocardium. This is frequently done by suturing the electrical lead in place to the heart muscle using conventional suture materials. The additional sutures required for anchoring may increase the trauma to the heart and may complicate removal of the pacing lead. Another method of anchoring is the use of helical suture structures affixed distally to the pacing lead. For example, U.S. Pat. No. 4,341,226 discloses a temporary lead for cardiac pacing or monitoring purposes. The lead has a curved needle having an attached surgical suture with a proximal helix molded into the suture immediately distal to the pacing lead. The helical suture and pacing lead are placed within the myocardium and the helical suture serves to anchor the lead within the myocardium. The helical anchor suture is typically nonconductive and this type of cardiac pacing lead typically has less conductive surface in contact with the myocardium. Yet another method of anchoring a pacing wire involves inserting a pacing wire which is completely insulated into the myocardium. An excess length is pulled through the myocardium. The surgeon the strips the insulation off from opposite sides of a section of the wire forming two small wings of stripped insulation material. The surgeon then applies traction on the lead, thereby pulling the excess lead back into the myocardium until the stripped section of wire is in the myocardium. The wings of insulation remain exterior to the myocardium and serve as an anchor. The stripping is typically done with a scalpel blade. This method of anchoring has many disadvantages including the additional time necessary to perform the wire stripping procedure as well as danger to the patient and surgeon while attempting to strip insulation from a wire lead with a scalpel.
There are disadvantages to either of these methods of securing the pacing wire to the myocardium in that removal of the pacing wire may cause significant trauma to the myocardium. In addition, pacing leads such as those disclosed in U.S. Pat. No. 4,341,226 have a conductive section which is larger in diameter than the pathway formed by the surgical needle in the myocardium. This results in additional stress or trauma to the myocardium when the conductive section is inserted or removed.
What is needed in this art are new pacing leads which overcome the disadvantages of conventional pacing leads.